High-frequency translating apparatus



June I, 1948;. c, PETERSON 2,442,662

HIGH-FREQUENCY TRANSLATING APPARATCS 5 Sheets-Sheet 1 Filed April 15,1942 Mada/a f/ao Pa/c/if/a/ fiource INVENTOP .C; PETERSON AIW 0. NJ

grroe/vgy June 1, 1948. 1.. c. PETERSON.

HIGH-FREQUENCY TRANSLATING APPARATUS 5 Sheets-Sheet 2 Filed April 15,1942 RELATIVE TRANS/0M! TTA NCE 0F TETRODE A5 FUNCTION OF SCREENSPEED-TO-GR/D SPEED RAT/ 0 FOR VARIOUS INPUT TRANS/TANGLES.

I I l V 2.0 (Egfcmog spa-0 AT sent-4 LECTRON SPEED AT GRID INVENTOR L.C. PEERSON A TTORNEV June 1, 1948. L. c. PEQTERSON 2,442,662

HIGH-FREQUENCY TRANSLATING APPARATUS Filed April 15, 19.42

5 Sheets- Sheet 3 INVENTOR LCPETERSON ATTORNEY June I, 1948. L. c.PETERSON HIGH-FREQUENCY TRANSLATING APPARATUS 5 Sheets-Sheet 4 FiledApril 15, 1942 lNVE/VTOR .C. PETERSON A TTORNEV 5 Shets-Sheet 5 June E,194. a... c. PETERsoN HIGH-FREQUENCY TRANSLATING APPARATUS Filed April15, 1942 T ATTORNEY IN 5 N TOR By L .c. PETERSON Patented June 1, 1948HIGH-FREQUENCY TRAN APPARATUS Liss 0. Peterson, Madison, N. J., assignorto Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York" Application April 15, 1942,Serial No. 439,059

7 Claims. 179-1715) This invention relates to high frequency dischargedevices for the production, amplification, and conversion of ultra-highfrequency waves and particularly to devices of this character whoseoperation is in large part dependent on the inertia effects of movingelectric charges.

The principal object of the invention is to secure eflicientamplification of waves of ultrahigh frequency, for example in the3000-megacycle range, using moderately low voltage electron tubes.

An additional object of the invention is to secure efiicient control ofthese ultra-high frequency waves with comparatively simple structures.

An additional object is to provide means and methods for efficientoperation at ultra-high frequenciesof structures which are in somerespects conventional having been developed for use at much lowerfrequencies at which charge inertia effects may be neglected.

It is a feature of the invention that both charge density variations andvelocity variations of a stream of moving charges take part in theoperation of the apparatus. 'It is another featurethat the effects, atan output circuit, of both charge density variations and velocityvariations, imparted to a stream of moving charges at an input circuitare combined in a favorable phase relation.

It is anotherfeature of the invention that space-charge repulsion ordispersion effects, heretofore regarded 'as constituting unavoidablelimitations on the efiiciency of velocity variation devices, are foundto provide a source of unforeseen advantages and are turned to goodaccount.

It is another feature of the invention that the potentials andgeometrical dispositions of the electrodes of a device constructed inaccordance therewith are so correlated with signal frequencies, charge,transit time and space-charge repulsion effects that a transadmittanceis secured which greatly exceeds not only the transadmittance obtainablewith the best available apparatus at ultra-high frequencies, but alsothe transadmittances obtainablewith the same apparatusat lowfrequencies; 1. e., at frequencies such that inertia and transit timeeffects are of no importance.

The phenomena which take place in so- -called velocity variation devicesare commonly explained on the assumption that space-charge repulsioneffects may, to a first approximation at least, be ignored. In suchdevices an electron stream is projected in succession through an inputgap, a

- 2 drift space and an output gap. High frequency signal energy isimparted to the stream at the input gap, and it is commonly conceivedthat initially this energy appears wholly inthe form of velocitydifferences between electrons, without substantial differences in thedensity 'of" the stream from point to point thereof.". Under theseconditions the" fas'te'r electrons willobviou'sly tend to overtake theslower ones so that a grouping or bunching of electrons will result."'It' is conceived that the faster electrons may actually overtake andpass'the slower'ones so that the electron stream, were ittoexten'd overagreat distance, would appear characterized by regions of high electrondensity's'ep'arated by intervening regions of much lower electrondensity, each electron of the stream, howevenpreserving unchanged theactual velocity with which it emerged from the input" gap. It is pointedout in these explanations that the output circuit, in order that'fit'may efi'iciently abstract the signal frequency energy from' the stream,'shouldbe placed along the path of the stream at'apoint of maximumcharge density, preferably the first ofsuchpoints. It is recognized thatthis is an idealized picture which is not wholly borne out in'practiceon account of spacecharge dispersion of so-called debunching effects.flhesefefiects, which are due to the mutual repulsive forces betweenelectrons, are conceived to impede or retard 'the formation f electrongroups as above described with' a consequent alteration of'the optimumposition at which the output gap should beplaced and a reduction in theefficiency of the apparatus. Another theory is based 'on' the assumptionthat space-charge eifects are jcontrollingi'tliat due to the repulsiveforces between electron's 'the faster ones, as theybegintogovertake theslower ones, will, instead of passingthem, be reduced in speed 'asby'elasticj collision with an interchange of momentum. In accordance"With'this theory it is suggested that wavesof density travel along themoving stream, each particular electron undergoing a "movement which' iscompounded of a steady gve g 'e qeay and an oscillatory o eme t SHPiPQSPh W-P S fore, it is .suggested that the output pr sm should bepla'cedalong thepath of the streain' at a point where a density maximum,preferably the first density maximum, exists. The positions along thestream path ,at which these density maxima are believed to occur differfrom the positions of the density maxima which arisefrom 3 fastelectrons overtaking slow ones, space-charge effects being neglected.

In one of its aspects the invention is based upon applicants recognitionthat the true state of affairs is not in accord with either of the aboveidealized pictures but lies somewhere in between; i. e., some of thefast electrons pass unimpeded by the slower ones while others of thefast electrons are retarded by elastic collisions, so that the chargegrouping is in part due to the unimpeded progress of fast electrons byslow ones and in part due to the interchange of their momenta as theyapproach one another. Thus the problem is presented as to how best totake advantage of the charge grouping due to space-charge repulsionefiects without undue sacrifice of the charge grouping due to velocityvariation, and, if possible, to bring these two different effects intofavorable phase relation.

This problem is solved by the invention. In the solution the movingelectron stream is retarded and compressed in an axial direction in sucha manner that the electron groups are much more closely spaced in thevicinity of the output circuit than in the vicinity of the inputcircuit. Space-charge repulsion effects are progressively built upalongthe electron stream from its input end to its output end wellbeyond the point heretofore considered possible. As a result, not onlyare the two types of charge grouping due, respectively, to momentuminterchange and to passage of fast electrons by slow electrons, causedto be greatly in excess of what is possible in the absence ofretardation, but, in addition, these two effects are brought into afavorable phase relation so that they are cumulative.

The invention both in its mode of operation and in its specific detailswill be explained in conjunction with the following description ofcertain preferred embodiments thereof, taken in connection with theappended drawings in which:

Fig. 1 is a diagrammatic cross-sectional view of a comparatively simplestructure which, by reason of the electrode arrangement shown and thepotentials applied, embodies certain features of the invention;

Fig. 2 is a simplified explanatory diagram of parts of the apparatus ofFig. 1;

Fig. 3 is a vector diagram of assistance in understanding the mode ofoperation of the invention;

Fig. 4 is a set of curves depicting the performance of apparatusaccording to Fig. 1;

Fig. 5 is a simplified diagram of a modification of Fig. 2, in which aconstant current is injected into the input region, and Fig. 5a is amodification thereof;

Fig. 6 is a simplified diagram of a modification of Fig. 5 wherein thedrift space is provided with an auxiliary space-charge controllingelectrode;

Fig. '7 is a simplified diagram of another modification of Fig. 5showing a plurality of spacecharge controlling electrodes;

Fig. 8 is a simplified diagram showing the inclusion of a space-chargecontrolling electrode in a structure like that of Figs. 1 and 2;

' Fig.9 is a set of curves depicting the performance ofapparatusaccording to Fig. 8;

Fig. 10 is a simplified diagram showing the addition of an auxiliaryinput gap to the apparatus of Fig. 8; and

Fig. 11 is a diagrammatic cross-sectional view of apparatus embodying anumber of the features separately shown in others of the figures.

Referring now to the figures, Fig. 1 illustrates one form of apparatusembodying the principles of the invention as applied to a system inwhich a compact air-tight envelope serves to define an evacuated spacein which the working electrons may be thermionically produced andundergo their proper movements, the comparatively large cavityresonators which serve as tuned circuits being disposed outside of theenvelope. If preferred, the cavity resonators or other tuned circuitsmay be contained entirely within the envelope, though, since it is nowpossible to make substantially perfect metal-to-glass seals, thestructure shown is preferred on account of its compactness and the easewith which adjustments may be made. A cylindrical evacuated envelope I!)having ends tapered for the purpose of strength is, provided. Withinthis envelope there are mounted in axial succession a cathode 2, a grid14, a screen l6, and a collector anode Hi. The cathode may comprise aflat plate I2 suitably treated on its outer face with thermionicallyemissive material. It may be mounted on and supported by a metal sleeve28 which projects through the end wall of the envelope [0 to provide anelectric connection. Suitable means, such as slots 22 or regionsofreduced cross section may be provided to reduce the fiow of heat fromthe cathode to the exterior of the envelope and thus effect economies inmaterials and in the energy required to maintain the cathode at thetemperature of emission. The sleeve 20 protruding through the end wallof the envelope, may be sealed into the latter in air-tight fashion. Thecathode may be maintained at a suitable temperature for emission ofthermions by a heater element 25 mounted immediately behind it; andpreferably e bedded in a suitable refractory plastic material 26 whichsubstantially fills the mounting sleeve 28. This heater element 24 mayhave heating current fed to it from an external source 28 by way of aconductor 30 passing through the sleeve 26, the sleeve itselfconstituting the return current path.

Since the function of the cathode is merely to provide a stream ofelectrons which shall be comparatively uniform both in velocity and indensity, and whose density is substantial, any cathode constructionwhich meets these requirements may be employed.

The anode l8, whose function is to collect the electrons after theirhigh frequency energies are largely spent, may be similarly mounted inthe opposite endof the envelope I0 being supported, for example, on aconducting member 32 which is sealed in the envelope wall, over whichoperating anode potential may be supplied as from a suitable potentialsource 34.

Spaced along the path of the electron stream between the cathode l2 andthe anode l8 are disposed a control electrode or grid l4 and a screenelectrode 16. Each of these electrodes may be in the form of a wire meshscreen, a perforated plate, an array of slats or the like. The primaryconsideration dictating the electrode structure is that it shall act tothe least extent possible as an obstacle to the electrons of the stream,and yet behave as an electric shield of high quality, segregating thecathode-grid space l3 and the screen-anode space H, respectively, fromthe central grid-screen space 15. The grid [4 and the screen l6 may eachbemounted centrally in an aperture of a plate or disc l4, l6 ofconducting material which extends through the envelope wall [0 toprovide means of coupling these electrodes with an external circuit, for

example, a cavity resonator. Similar plates or discs l2, It may extendradially outward from the cathode sleeve 20 andthe anode support 32,respectively, to provide a similar coupling means with the appropriatecavity resonator.

Input and output cavity resonators may be connected to the electrodes ofthis structure in any suitable manner, the arrangement which ispreferred on account of its simplicity being that shown in the figure,wherein a first or input cavity resonator 36 is coupled to the cathodeI2 and the grid t l, while a second or output cavity resonator 38 iscoupled to the screen l6 and the anode l8. Blocking condensers 31, 39may be inserted at convenient points, for example, at the peripheries ofthe discs l2, 58, so that a steady potential difference may beestablished between adjacent electrodes without substantially reducingthe efiectiveness of the resonators at the signal frequency. To this endthese condensers should be of capacitance values such as to present buta negligible impedance at the operating frequency. They may convenientlybe formed by providing adjacent portions of the plates with parallelflanges, a thin annular strip of insulating material being placedbetween. With this arrangement the space i5 between the grid Id and thescreen It is substantially free of signal frequency fields, and to thisextent is similar to the so-called drift space of velocity variationdevices. However, it is by no means free of steady fields.

Signal energy may be supplied to the input cavity resonator 36 from ahigh frequency signal source which is symbolically represented by thegenerator d!) by way of a coaxial line 4! to a coil or loop 42 whichextends within the cavity 36 through a hole in the cavity wall to link asmall amount of the magnetic field within the cavity. Signal energy maybe withdrawn from the system by a similar loop 44 similarly coupled by aline E5 to the output resonator 38 and supplied to any suitable load,symbolically represented by the resistor 46. The resonators 36, 38 maybe tuned by adjustment of the position of conducting rings d8, 5%]. Asis well known, when resonators of this type are excited, high frequencyelectric fields exist across the input region l3 defined by thereentrant cathode I2 and the grid I4 and also across the output regiondefined by the screen l6 and the anode l8. These electric fields areestablished in a substantially axial direction, both of the envelope l0and of the electron stream flowing from the cathode l2, through the gridi4 and the screen [6 to the anode I8. They interact with the electronsof the stream in their movements between these electrodes and acrossthese regions to cause variations in their energies.

It is preferred that the electron transit angle across the input regionl3 should be fairly long, i. e., of the order of a full cycle of thesignali frequency or more, and also that this input region 53 besubstantially space-charge limited, i. e., that the velocities andaccelerations of the electrons at the cathode surface be substantiallyzero, the space-charge density within this region having a high value.

The output region I! defined by the screen l6 and the anode I8 ispreferably of a short electrical length. This result may be achieved bymaking its geometrical length short and the anode potential high or byany compromise between these conditions. As shown, the space H isconsiderably shorter than the input space l3; while the anode potentialis substantially in excess of the potential of any other electrodesLength of cathode-grid space inches 0.06

Length of grid-screenspace do 0.12 Length of screen-anode space do 0.03Cathode potential volts' 0 Gridpotential do Screen potential do 15 Anodepotential do 400" These potentials may be applied to the, electrodes ofthe device in any suitable manner-,as by connections to asuitable source34. Since the potentials of the grid l4 and the screenlfij' are fairlycritical, means may be provided for experimental adjustment of thesepotenials tovcorrect values, as .for example by a potentiometer 3|connectedacross at least apart of the source 3.4 and movable taps 33, 35which are connected; respectively, to the grid Id and to the. screen 16;

In: the-operation of the apparatus, electrons,

thermionically emitted by the cathode 12 are.

drawn toward the grid hi, pass through. the :lat-

terat considerable velocities and. travel at. con-..

tinually reducedvelocities to the screen, l6, pass through the screenand are immediately acceler-' atedby the anode potential to cross thescreen anode space I? with great, rapidity and are finally collected bythe anode l8 and returned through the potential source 34 tothe cathode.I2; Duringsuccessive half: cycles of the signal applied to the loop dithe signal frequency electric field which is established between thecathode lZ-and the grid Hi is successively added to and subtracted fromthe steady electric field which appears between these electrodes due tothe applied steady bias potentials. In the case of tubes withclose-spacing between the cathode and grid, with the consequence thatthe transit angleacross the input gap is small it follows that atinstants when the fields are additive, electrons pass through the grid Mwith greater than average velocity and at instants when it subtractsthey pass through with less than average velocity. Associated with thesevelocity variations there also exists a variation in the number ofelectrons passing the grid, i. e., in the charge density of the stream.Thus, in general, the grid efiects both velocity variation and densityvariation of the electron stream, though these two sorts of stream.

variations are by no means necessarily or usually in any. favorablephase relation. In the case of tubes with greater spacing between thecathode and grid with the consequence that the transit angle across theinput gap is not small, the simple phase relations described above nolonger apply. Nevertheless, the general effect is the same when properaccount is taken of the phase shifts, and the grid. again affects bothvelocity variation and density variation of the electron stream.

In conventional. velocity variation devices: elec: trons whosevelocities have been varied in accordance with the signal are nextallowed to traverse a field-free drift space in which the fasterelectrons tend to overtake the slower ones so that, at a distance fromthe grid, they will have become partially segregated into groups orbunches. At the beginning of the bunching process the forward electronsof each bunch are the slowest and the rearward ones the fastest, whilethe bunch as a whole travels with the mean or normal speed of thestream. It has been conceived by some that, ideally, the fasterelectrons eventually overtake and pass the slower ones and proceed inturn to catch up with the slower electrons of the next group ahead, sothat the proper point at which to place the output circuit, i. e., thepoint of maximum charge density, is the point at which the fastelectrons have overtaken but not yet passed the slower ones.

It has been recognized that as this process proceeds and the density ofthe bunches becomes greater and greater, the completeness of thebunching process will be impeded, delayed, and reduced by mutualrepulsive forces between electrons; i. e., a space-charge dispersion orso-called debunching effect will come into play. However, this has beengenerally regarded as indicating merely that the output circuit shouldnot be placed exactly at the point dictated by elementary considerationsbut rather at a point somewhat further along the stream path.

A different mode of operation for such devices is proposed in W. C. HahnPatent 2,240,183, April 29, 1941, wherein the space-charge dispersioneffects are treated as controlling. It is there suggested that as thefaster electrons tend to overtake the slower ones they will beprogressively retarded by the latter, the slow ones in turn beingprogressively accelerated by the faster ones. On the assumption that thephenomena are elastic in nature an interchange in momentum is said tooccur, the roles of fast electrons and slow ones being interchanged, thefinal effect being that of a density wave traveling along the path ofthe stream. An analogy is drawn to a rubber rod along which elasticwaves move back and forth while the rod is being translated as a wholein the direction of its length. Thus, according to the teachings of theHahn patent there are at any instant a succession of regions of maximumdensity in which the velocity is low, interspersed with regions ofreduced density in which the velocity is high. Each successive densitymaximum is conceived to be like the others and it is suggested that theoutput circuit may be placed to coincide with any one,

Now it is a fact that neither of these two modes of conceiving theoperation of devices of this character is complete. There is, in fact,always some elastic interchange of momentum while at the same time someof the fast electrons which find themselves between two density maximaor groups succeed in passing through the group ahead of them from rearto front as though spacecharge repulsion effects were not present. Thesetwo different effects tend, in general, to offset each other so that theresults obtainable in the past with devices of this character arenotorious- 1y inferior to those predicted by either theory.

In accordance with the invention, these two effects are so controlled asto aid each other instead of opposing each other while at the same timeeach one separately is caused to be substantially greater than it couldbe if it stood alone. These results are secured by the propercorrelation of the electrode spacings and potentials with the spacecharge present in the de-' vice in'the manner now to be explained.

For the sake of simplicity of explanation, let it be assumed that theapparatus is a plane parallel electrode arrangement such as thatdiagrammatically depicted in Fig. 2, wherein a cathode 2, a grid l4, ascreen 5, and an anode l8 serve the purposes of the similarly designatedelectrodes of Fig. 1. If it be assumed further that the total length ofthe beam from the cathode to the anode is small as compared with itsdiameter the effects of radial spreading may be neglected. Theseassumptions are simplifying idealizations which, however, are not farfrom the reality as it exists in a practical embodiment such as thatshown in Fig. 1. In the figures of the drawing, the spacings between theseveral elements of the tube structure are shown extended for the sakeof clearness. It is evident that they may be shortened. in relation tothe diameter of the elements in order to maintain a more homogeneouselectron stream or may even be lengthened provided that undue dispersionof the beam does not result. a

In Fig. 2, as in the other simplified diagrammatic figures, namely Figs.5, 6, '7, 8, and 10, batteries are shown to indicate the preferredpotential differences between the more important electrodes, cathode andanode potential supply sources being omitted in the interests ofsimplicity.

Consider a velocity-varied electron stream of comparatively high averagedensity issuing from the control grid M of Fig. 2. Since the screenelectrode I6 is maintained at a potential, measured with respect to thecathode l2, which is considerably less than that of the grid [4, anelectric field exists between these electrodes of a character such as toretard the electrons of the stream. With a screen potential of onlyone-fourth of that of the grid potential, the retarding field is such asto reduce the electron velocities at the screen 16 to one-half of theirvalues at the grid. As a result, in the absence of an alteration in beamcross section, the average beam density at the screen I6 is twice, andthe average electron spacing one-half, of What these quantities are atthe grid IA. The stream has been subjected to a progressive axialcompression. It is under these new conditions that the bunching processis now to take place.

Without necessary subscription to any particular theory, it is felt thatthe bunching phenomena which take place under these new conditions aresomewhat as follows:

Consider a region of the stream path at which bunching has justcommenced. In the case of a signal input of the purely velocityvariation variety, this point may be a point somewhat removed from theinput means, i. e., the grid M of Fig. 1. In the case of combinedvelocity variations and density variations, this point may beimmediately to the right of the grid. At this point, wherever it may be,the incipient groups of electrons are spaced apart by a distancedetermined by the mean speed of the stream and the signal frequency,while the actual velocity of each electron is compounded of a steady orD, C. component and a signal frequency or A. C. component. The groupsthemselves are advancing with a group velocity equal to the averagevelocity of the stream. An electron lying between two adjacent incipientcharge groups has, in general, a speed which differs from the averagestream speed. Suppose this electron to be somewhat ahead of themid-point between two adj-acent'incipient charge groups. It. willnormally be traveling with a velocity greater than the average streamvelocity and at the same time it will be repelled by the charge groupahead of it more strongly than it is urged forward by the charge groupbehind it. These repulsive forces offset each other only for an electronwhich lies midway between the two charge groups and which may have avelocity which is forward or backward with respect to the group,depending on the phenomena which have taken place in the input region.These repulsive forces represent potential energy, which is reduced to aminimum when the electron is equally spaced between adjacent chargegroups. At the same time, by virtue of the alternating components of itsspeed and momentum, the electron under consideration has kinetic energyrelative to the stream as a whole. These potential and kinetic energiesare related in such a way as to promote a tendency for the electrons tooscillate to either side of some point lying between the two adjacentcharge groups, for example, the mid-point, and this energy ofoscillation will have a definite value. To a first approximation thisoscillation energy is given by where am is the maximum value of theoscillatory excursion of the electron from the point about which itoscillates and K is an equivalent elastic constant which, again to afirst approximation, is proportional in a plane parallel electrodearrangement such as that here under discussion to the average density ofthe beam.

Under these conditions, the amount of bunching or groupin associatedwith the particular electron under consideration is related to thedifference between the distance separating it from the nearer group andthe distance separating it from the farther group.

Consider next the modification of this state of affairs which existswhen the stream as a whole has been reduced in velocity, its densityincreased and its group spacing reduced by the influence of theretarding field which is established between the grid M and the screenIt. If no signal frequency energy is abstracted until after the beamenters the output region ll defined by the screen i and the anode l8,then, at the entrance plane of this output region, i. e., just as thestream is about to enter the screen iii, the energy of oscillation hasthe same value as before. In other words, the oscillation energy isconserved during the travel of the stream along the drift space andagainst the retarding field which is established therein. Assuming, forexample, that the electrode potentials have the values recommended forFig. 1, namely, the screen potential is one-quarter of the gridpotential, then the average velocity of the stream at the plane of thegrid It will have been halved; its mean density will have been doubledand the spacing between adjacent electron groups will have been halved.The equivalent elastic constant will be twice its prior value. Withconservation of the oscillation energy this requires that theoscillatory excursion of the electron on either side of its position ofequilibrium he reduced by the factor /2. Since, however, the spacingbetween adjacent groups has been reduced by a factor 2, it is easilyseen that a substantial enhancement or increase in the 10 amount ofbunching or grouping will have resulted; in other words, the ratio ofthe peak or maximum value of the electron stream density to the averagevalue is greater, where the beam has been axially compressed, than thesame ratio at the input gap.

The electron stream with its electron bunches or groups, substantiallyenhanced in the manner described above, now passes through the screenand travels rapidly across the output region ll defined by the screen [6and the anode Hi. The anode potential is preferably maintained at arelatively high value such that, despite the comparatively low electronvelocity at the screen It, the electron transit angle in the outputregion ii is but a small fraction of a cycle at the signal frequency.The passage of the segregated electron groups across this output regiongenerates and maintains an electromagnetic field within the outputcavity resonator 38 in well-known manner, from which signal frequencyenergy, greatly amplified by the apparatus described above, may beabstracted by the loop 46 and delivered to the load 46.

If it were not for the phenomenon of space charge the electrons of thestream would continue throughout the length of the drift space withoutmodification of the original velocities with which they emerge from gridM by reason of the presence of other electrons or electron groups withinthe space. With these electrons grouping or bunching would take place ona purely kinematic basis, i. e., unaffected by intercharge repulsions.In apparatus of conventional design this kinematic grouping is offsetand impeded by the space-charge repulsion grouping. By proper .selectionof electrode spacings and operating conditions in accordance with theprinciples of the invention these two effects are both increased inmagnitude and rendered cumulative.

The reference in the above explanation to es! cillations and oscillationenergy is not to be taken as implying that a number of full oscillationcycles necessarily take place in the electron stream between the gridand the screen. There may be a number of such full cycles or there maybe only a fraction of a cycle, depending on the detailed arrangement ofthe electrodes and their potentials. In the particular case discussedabove, it is believed that there is but one density maximum; i. e., theelectrons, while. they have oscillatory energy in the sense that theyare masses in motion under the influence of restoring forces, fail tocomplete the first full cycle of such oscillation and are, so to speak,caught by the output circuit just as they are about to be reflected forthe first time. In other more elaborate structures constructed inaccordance with further features of the invention, they may be caught onthe second or third reflection, or even after a still higher number ofreflections. The limiting consideration here is that if the averagevelocity of the stream be too greatly retarded and the stream be axiallycompressed with too great rapidity, electrons may actually be brought toa zero velocity at some point of the stream, in which case someelectrons may. be turned back relatively not only to the moving chargegroups but also to the electrodes, with the formation of a virtualcathode and a sudden large reduction in the stream current. Thisphenomenon is fully discussed in an article pub.- lished in the BellSystem Technical Journal for July 1939 at page 465. It is recommendedthat operation of apparatus in accordance with the 11 invention be keptwell to the safe side ofsuch a condition.

When the phenomena described above are treated mathematically, it can beshown that the transadmittance of the device as a whole may be expressedin the following form:

where the symbols all refer to measurements made before any alternatingcurrents are impressed on the device, that is, to measurements made on adieot current basis and v1 and 122 are the steady components of thestream velocities at grid and screen, respectively;

91 and 02 are the transit angles across the input region and the driftspace, respectively,

5' is a space-charge factor for the drift space,

where T is the actual transit time across the drift space and To is thetransit time in the absence of space charge, both measured'when noalternating currents are flowing,

go is the transadmittance of the structure at frequencies so low thattransit time effects may be neglected, A is a constant which depends onthe permeability to electrons of the grid and the screen,

6 is the base of naperian logarithms, and

The procedure employed in arriving at the above mathematical expressionis laborious, though straightforward. In Electron Inertia Effects by F.B. Llewellyn there are given general electronics equations for dischargedevices having parallel plane electrodes. Solution of these equationsfor the cathode-grid space provides boundary conditions to be insertedin the coefficients of the same equations for the gridscreen space.Solution of these, in turn, provides boundary conditions to be insertedin the coefficients of the same equations for the screenanode space.Solution of these last equations ives the output signal frequencycurrent in terms of a large number of factors, among which are theelectrode spacings, the electrode potentials, and the electron steamdensity. Division of the output signal current by the input signalvoltage gives the transadmittance.

To simplify the work, various approximations may be made. Thus, inarriving at the above expression it has been assumed that in the inputregion I3, space charge is high and the transit angle of the order of afull cycle or more; that in the output region the space charge isnegligible and the transit angle but a small fraction of a cycle.

In the expression (1) the first two terms represent the efi'ects ofdensity variations imparted to the stream in the input region and thethird term represents the effects of velocity variations imparted to thestream in the input region, After multiplication of the third term bygo, it may be rewritten in the form tion practice and is discussed in anarticle by Hahn and Metcalf published in the Proceed gs of the Instituteof Radio Engineers for February 1939 at page 106. In this factor themodification consists in the replacement, in the denominator, of thedrift tube potential by the geometrical mean of the potentials of thescreen and the grid, i. e., of the electrodes which define the drift.space. The second variable factor, namely,

may be termed a modulation factor and relates the alternating currentcomponent of the velocity at the grid to the alternating component ofthe:

potential applied between grid and cathode.

It is of interest to note that the first two terms:

of the above expression (1) for the transadmittance are independent ofthe frequency of the applied signal, while to a first approximation thelast term is equally so. This means that the apparatus whose performancethe expression describes is useful over a wide frequency range and,within this range, shows great frequency stability. This is true as longas the frequency is within the range in which the restrictiveassumptions on which the expression'is based are satisfied; that is, aslong as the frequency is so high that the transit angle across the inputspace is long and yet not so high but that the transit angle across theoutput space may still be short.

Regarding the three terms of the above expression (1) as vectors orvcomplex members it will be observed that the first two terms are purereal numbers while the third term is a complex quantity whose phaseangle depends solely on the transit angle across the input space. Thusthese vectors may be given any desired phase relation merely by suitableadjustment of this input transit angle. They may be brought into phasecoincidence by adjusting the input transit angle to an integral numberof full cycles; they may be brought into phase opposition by adjustingthe input transit angle to an odd number of half cycles. It ispreferred, however, to adjust the input transit angle approximately toone or other of the members of the sequence 1%, 2%, 3 A, etcncycles,preferably the first member of the sequence. As explained in F. B.Llewellyn Patent 2,190,668, February 20, 1940, with this relation theinput circuit of the apparatus, which is essentially a plane paralleldiode, presents to the circuit to which it is coupled an impedancehaving a negative real part, i. e., a negative resistance. Such anegative resistance compensates, in part, for stray losses in the systemand is believed to represent a desirable operating condition, as long asthis result can be obtained without undue sacrifice. With an inputtransit angle of 1%, etc. cycles, the phase angle of the vectorrepresenting the third term of the above expression is 45 degrees.Referring to Fig. 3, in which the three vectors in question are shown inthe complex plane, it will be observed that the resultant vector R isunder these conditions only slightly reduced in magnitude as comparedwith its value when the input transit angle is a whole number of cycles,in which case all of the vectors lie alon the real axis.

In Fig. 4 there are plotted a group of curves which exhibit the absolutevalue of the vector sum represented by the expression 1) under differentconditions, i. e., as functions of the ratio of the velocity at thescreen to the velocity at the grid. These curves are all taken for astructure such as that shown in Fig. 1, in which the screen-to-gridspacing is twice the cathode-to- 13 grid spacing. The parameter whichdistinguishes between these curves is .the magnitude of the inputtransit angle. It will be observed thatfor properly chosen velocityratios, .the 'relativetransadmittance, i. e., the ratio of the highfrequency transadmittance to the low frequency transadmittance, go, .isquite large and that the high frequency transadmittance is thus manytimes greater than the low frequency transadmittance.

In the apparatus of Fig. 1 the drift space 15 between screen and thegrid is substantially de void of signal frequency fields, the'outputtuned circuit, i. e., the resonator 38, being connected between thescreen &6 and the plate l8 while the input tuned circuit or resonator.36 is connected between the cathode i2 and'the grid .14. Thissimplifies the discussion and the analysis.

The apparatus of Fig. 1 may be rendered regenerative or self-oscillatoryby feeding back a portion of the energy of theoutput circuit to theinput circuit in proper phase. Expedients are well known for effectingthis result. For example,

it may be accomplishedmerely by auxiliary loops 58, 60 introduced intoeach of the cavity resonators 36, 38 at the proper points, the loopsbeing coupled together by appropriate means as through a coaxial line1-32 whose length is adjustable as by a trombone-like sliding member 63.

It will be observed that the characteristic curves of Fig. 4are-exceedingly steep in .thegeneral region in which .the electron speedat the screen is low relative to that at the grid. This exhibits thefact that the apparatus of Fig. 1 may serve excellently as a converter.of oscillations, for example, either a detector or a modulator. It mayconveniently be operated as a modulator by applying a signal "of lowerfrequency than that of the principal signal derived, for example, from asuitable source 51 to the screen electrode it or, indeed, in anyother-desired fashion.

The principles of the invention are equally applicable to variousmodified arrangements of electrodes and of bias potentials of whichFigs. 5, 6 and '7 show some in simplified schematicform, a number ofthem being shown additionally incorporated into a single structure inFig. 11.

Fig. shows a simplified view of an arrange ment of four grids disposedinsuccession along the path of the electron stream from thecathode l2 tothe anode iii, the input tuned circuit, for example a cavity resonator36 being connected between the first grid 19 and the second Hi, whilethe output tuned circuit or resonator .33 is connected between the thirdgrid it and the fourth 12. With this arrangement there is no signalfrequency field between the cathode l2 and the first grid Hi andsubstantially none between the last grid 12 and the anode .18. Thecathode 12 thus serves merely as a source of electrons .and the anode l8merely as a collector of spent electrons. The first grid It] should bedisposed at such a distance from the cathode i2 and maintained at such apotential that a dense beam of fairly high velocity electrons reachesthe first grid 19, the cathode being space-charge-limited without theformation of any virtual cathode at any point along the beam. The spacebetween the first grid is and the second id constitutes the inputregion, signal energy being supplied as through a coupling loop 42coupled to the input cavity 36. The space i5 between the secondgrid I 4and. the third l5 constitutes the drift space. The electron stream isboth velocity varied and density varied as it emerges from the secondgrid I4 and starts its travel through the drift space 115.

grids Iii, it may be maintained at like poten- '14 It .is thenconsiderably .reduced in velocity and subjected to a progressive axialcompression in the drift space .[5 in order to reduce the spacingbetween adjacent charge groups, as by a retarding field in the .mannerdescribed in connection with Fig. l. The fourth grid 12 may be disposedfairly close to the third grid l5 and maintained at a comparatively highpotential in .order that the transit angle across the output regiondefined by the third grid 15 and the fourth 12 may be short. Afterpassing across the output region the electrons of the stream, from whichthe signal frequency energy has been largely abstracted as b the loop 43, travel to the anode l8 and are there collected. The first and secondtials in which case except that the electron stream arriving at the gridll! contains no density variation component, the .action and performance.of the apparatus are substantially as described above in'connectionwith Fig. 1 and a transadmittance enhancement of comparable magnitudemay be obtained.

Fig. 5A shows a modification which may be the same asFig. 5 except forthe connection of the output cavity resonator between the screen It andthe anode H] as in Figs. 1 and 2. Thus the advantages of the simpleroutput arrangement of Figs. 1 and 2 are combined with the advantagesobtainable from constant speed injection as .in Fig. 5. In otherrespects this apparatus maybe similar to Fig. i, and corresponding partsare similarly designated.

Still further enhancement of the transadmittance may be gained bymaintaining the second grid it of Figs. 5 and 5A at .a reduced potentialwith respect to the first grid '10 so that a retarding field existsacross the input region itself as well as across the drift space. Inorder that the first and second grids may be maintained at differentpotentials without having the resonant cavity which is connected to themshort-circuit the input signal voltage, ablocking condenser 31 ofappropriate capacitance value may be included in the circuit. The sameconsideration dictates the use of a blocking condenser 39 in thecircuits of the third and fourth grids. These blocking condensers may beformed as described above in connection with Fig. .1 or in any othersuitable manner and should have capacitance values such that theypresent negligible impedance at the signal frequency.

Still further improvement is obtainable by the division of the driftspace l5 between the input region and the output region into two or moreparts by the interposition of additional grids therein, which gridsshould, in accordance with the principles of the invention, bemaintainedat reduced potentials with respect to the grids which definethe input region, and serve to regulat .the space charge density intheir vicinity. Fig. '6 shows in simplified diagrammatic form anarrangement for constant injection velocity as in Fig. 5 with oneadditional space charge controlling grid 78 in the drift space. Fig. 7shows an arrangement which is the same except for the fact that twointervening space-charge-controlling grids as, '82 are disposed in thedrift space. Fig. 8 shows an arrangement which resembles that of Fig. 2but in which an additional space charge controlling grid Ed isinterposed between the control grid l4; and the screen grid 16. Variouspermutations and combinations of these arrangements are entirelyfeasible within the scope of the invention.

In the arrangements of Figs. 6, '7 and 8, respectively, the featurewhich is common to all of them is the interposition of one or morespace-chargecontrolling grids in the drift space between the inputregion and the output region. Such grids provide additional sources ofcontrol of the change in average beam velocity and the amount of itsaxial compression. The exact nature of the improvements resulting fromthese modifications are not fully known although tests have shown thatthey exist. Without unqualified subscription to any particular theory itmay be suggested that, in a broad sense at least, the nature of theseefi'ects is as follows:

Referring again to Fig. 4 which represents the characteristic curves ofapparatus having the electrode arrangement shown in detail in Fig. 1 andschematically indicated in Fig. 2, i. e., devoid of auxiliary spacecharge controlling grids in the drift space, it will be observed thateach of these characteristic curves rises more and more steeply as thestream velocity at the plane of the screen electrode is reduced up to acertain point at which the curves terminate abruptly and for which thespace charge factor g, which was defined above in connection withEquation 1, reaches a value of unity. These terminal points representunstable operation and correspond to conditions in which the electronvelocity at some part of the stream path has been brought so low that avirtual cathode is formed with a resulting sudden reduction in thestream current. These phenomena are fully discussed in an articlepublished in the Bell System Technical Journal for July 1939 at page465. In view of this instability it is advisable to operate the deviceunder conditions corresponding to a lower point on the characteristiccurve such that the maximum signal frequency peak swing will not passthe threshold of instability.

When, on the other hand, one or more spacecharge-controlling grids areinterposed on the drift space, the relative transadmittance curves maypass through a maximum value as shown in Fig. 9 before the instabilitypoint is reached. The origin of the phenomena which gives rise to themaximum values of these curves is believed to be related in a somewhatinvolved way to the phase relations between the velocity variationcontribution and the density variation contribution to the total currentin the successive sections of the drift space and also to the fact thatwhen the axial beam compression is given two different values indifferent parts of the drift space, alterations may take place in therelative amounts of charge bunching due to drift action and of chargebunching due to space charge effects (in less precise but more graphiclanguage, grouping due to coasting and grouping due to elasticrepulsions). Whatever the intrinsic nature of the phenomena, it resultsthat with the interposition of one or more space-charge controllinggrids in the drift space the characteristic curves of the resultingapparatus exhibit maximum values such as those shown in Fig. 9. It isobviously desirable that the peak or maximum value be selected as anoperating point for the apparatus when used as an amplifier, since underthese conditions it will be much less sensitive to small changes inelectrode biases than when operated on a steeply sloping characteristic.On the other hand, when operation as a converter, a detector or amodulator is required, for which a steeply sloping characteristic isdesirable, a suitable alteration in the bias potential 16 of one of thespace charge controlling electrodes immediately shifts the operatingpoint from the maximum peak to the sloping part of the characteristicand transforms the apparatus from an amplifier into a converter.

The relative transadmittance curves of Fig. 9 are plotted for apparatushaving the electrode configurations and spacings shown in Fig. 8, inwhich the drift space l5, I5" is four times the length of the inputregion l3, a single spacecharge-controlling grid 84 being placed at themid-point of the drift space, the output region being electrically shortas before. To facilitate the analysis the third grid l5 was taken asbeing maintained at the same potential as the first grid I4 so that theelectron velocity of entrance into the output region I! was the same asthe velocity of exit from the input region l3, being reduced elsewherein the drift space by the action of the space-charge-controlling-grid84. The curves show the transadmittance as a function of the ratio ofthe electron velocity at the space-chargecontrolling grid 84 to thevelocity at the control grid l4. As in the case of Fig. 4, which showscorresponding curves for the simpler structure of Figs. 1 and 2, theparameter which distinguishes the curves is the transit angle across theinput region. It will be noted that the optimum conditions do not difiergreatly from those for the apparatus of Fig. 1, i. e., the velocityratio for which the transadmittance maximum occurs is approximatelyone-half. Thus the bias potential of the space-charge-controlling grid84 should be adjusted to approximately one-quarter that of the controlgrid l4. However, it is by no means necessary that the specificlimitations under which the curves of Fig. 9 were obtained shall besatisfied, and various modifications of the spacings, withcorrespondingly different values for the potentials of the space chargegrid 84 and of the output grid it relatively to the control grid [4 mayprofitably be employed.

As with the apparatus of Fig, 1, it is equally the case with theapparatus diagrammatically shown in the other figures that adjustment ofthe input transit angle, for example, to a value of 1%; cycles, resultsin an optimum value of input loading, i. e., the system presents anegative input resistance to the circuit to which it is coupled. It isalso true that this result may be secured with but a small reduction inthe overall transadmittance of the device from the maximum obtainablevalue, namely, that secured when the velocity variation vector of Fig. 3is exactly in phase with the density variation vector.

Still further advantages result from the introduction of a furtherfeature of the invention by which optimum values of input loading may beobtained without any sacrifice in transadmittance. To this end,provision is made for effecting a preliminary adjustment in the phasedisplacement between the density variation signal and the velocityvariation signal at the input region itself. Thus Fig. 10 shows anarrangement which is similar to that of Fig. 1 with the exception of thefact that a short velocity variation gap 8': defined by two adjacentclosely spaced control grids I4, 86 is added following the principalinput region l3. The second grid 86, defining the out put plane of thevelocity variation gap 8? may be maintained at the same potential as theprincipal control grid l4 so that the associated auxiliary tuning cavity88 may be a continuous conducting surface, while, in view of thepotential difference between the cathode l2 and the first grid M, ablocking condenser 37 must be interposed in' the wall of the principalinput tuning cavity 36 as shown. The signal is applied to the principalinput gap is by way of a coupling loop 42 in the manner heretoforedescribed and, in addition, it is passed through a phase-shifting device9%! of any suitable type and by way of an auxiliary loop 92 to theauxiliary velocity variation gap 8?. Thus the electromagnetic field inthe second cavity 88 is out of phase with that of the first cavity 36.The common cavity wall should be of high conductivity and the secondgrid It should preferably be a substantially perfect shield in order topre vent deleterious interaction between the fields. With thisarrangement the phase displacement between the velocity variationcomponent and the density variation component in the electron stream asit emerges [from the auxiliary gap may be such that, even when thetransit angle across the principal input gap l3 has the desired value ofabout 1 cycles, the effects of both of these variations at the outputgap I! may be brought into phase coincidence so that they stand in adirectly additive relation.

The features hereinabove described are mutually compatible and Fig. 11shows a structure embodying a number of them. Referring to this figurethe envelope iii, the cathode structure and the anode structure may besimilar to those shown in Fig. 1 and each of the grids may be of similarconstruction and similarly mounted. The grids shown have the followingfunctions. Starting from the cathode H in the direction of theprojection of the stream, the first two grids in order, Iii, i l definethe principal input region which, as before, may be of such a length andits grids at such a potential that the electron transit angle across itis 1 4 cycles. This part of the system is arranged for constant velocityinjection into the input region as in Figs. 5, 6 and 7 although it mayequally be arranged in the manner shown in Fig. l. A resonant cavity 38is coupled to these two grids and, in order to provide for a retardingfield across this input region a blocking condenser is provided inseries with the circuit formed by the cavity walls, for example, by aninsulating band 31 in the ring 48 which closes the cavity 36.

Following this input region is a second much shorter region or gap 81defined by the second and third grids I4, 86 which, as shown, may bemaintained at the same potential, and to which is coupled another cavity88 tuned to the same frequency as the first cavity 3i). The signalderived Ifrom a suitable source 40 is applied directly to the principalinput cavity 36 through a coupling loop 42 in the manner heretoforedescribed and is also fed through a phase shifting devices!) of anysuitable type to the second cavity '88. As in the case of Fig. 10 thisarrangement provides means for an initial adjustment of the phasedisplacement between the two current components prior to entry of thestream into the drift space.

After emerging from the auxiliary input gap 81 the beam travels througha drift space toward the output gap and, in doing so, passes through twosuccessive space charge controlling grids 80, 82. These grids may bemaintained at like potentials so as to define a field-free space betweenthem, but it is preferred to maintain the second grid 32 at a slightlyreduced potential with respect to the first grid 80- in order that asubstantial amount of axial beam compression may take place in eachsection of the drift space.

The output region defined by thelast two'grids l6, 12*should'be of'shorttransit angle as before and the two grids which define it may bemaintained at such potentials as will produce the greatest possibleamount of energy abstraction within this-output region in accordancewith the principles heretofore disclosed. The anode l8 serves to collectspent electrons.

For-operation of the above-described apparatus as a modulator, detector,or other converter, an auxiliary signal source 96 of lower frequencythan that of the ultra-high frequency sourced!) may be connected to anysuitable electrode, for example to the first of the space chargecontrolling grids 89 of Fig. 11. Variousother points of-application ofthis source are equally possible. It will be understood that foroperation in this manner it is desirable so to adjustthe potentials thatthe operating point lies on the sloping portion of the transadmittancecharacteristic.

Variations and departures from the particular details described above byway of illustration will suggest themselves to those skilled in the artas being within the scope of the invention as defined by the appendedclaims.

What is claimed is:

'1. In anultra-high frequency discharge device of thetype in whichtransit time efiects play -'a controlling part, the combination whichcomprises means for projecting astreamof moving charges-along aprescribed path; signal input means-disposed-at a point of said path forimparting charge density variations and velocity variations tothecurrent of said stream, a region disposedalong said path andfollowing said input means in which there-occurs-a grouping ofsaid'charges due to said imparted variations, an electrically-conductivemember in said region for establishingan electric field, an electricallyconducting path connecting said member to the stream projecting means,said pathincluding a source of steady polarizing electromotive force ofsuch magnitude and polarity that the resulting electric 'field of theelectrically conductive member retards .without reversing all ofsaid-charges, signal output'meansdisposed following said-region at a'point along said path at which the velocities of said chargeshave beenreduced by said field, and means for adjusting thepotential of saidinput'means to a value such that the input impedance of said device hasa desired value.

"2. In an ultra-high frequency discharge device of the'type in whichtransit time effects play a controlling part, the. combination whichcomprises-means -for projecting a stream of moving charges along aprescribed path, signal input means-disposed at a point of said path forimparting-charge density Variations and velocity variations tothe-current of said stream, a region disposed along said path andfollowing said input-means-in which there occurs a grouping of saidcharges due to said imparted variations, means within said'region forreducing the velocities of all of the moving charges by substantiallythe same amount to cause the charges to attain a mean velocity of the,order of one-half that with which they entered the region whereby theaverage charge density of said stream is doubled with respectto thedensity with which it'entered the region, signal output means disposedfollowing said region at a point along said path at which the velocitiesof said charges have been reduced-by said field, and-means for adjustingthe potential of said-"input" means-to a value such 19 that streamcurrents at said output means due to said charge density variations beara favorable phase relation with respect to stream currents at saidoutput means due to said velocity variations.

3. In a ultra-high frequency discharge device of the type in whichtransit time effects play a controlling part, the combination whichcomprises means for projecting at high velocity a dense stream of movingcharges along a prescribed path, signal input means disposed at a pointof said path for imparting velocity variations and charge densityvariations to the current of said stream, signal output means forconverting each of said variations into an output current component,means positioned between said signal input means and said signal outputmeans for reducing the velocities of all of said charges bysubstantially the same amount in order to efiectively double the averagecharge density of said stream, and means for adjusting the transit angleacross said signal input means to bring the output current componentsresulting respectively from the velocity variations and the chargedensity variations into cophasal relation, whereby said output currentcomponents are cumulative.

4. The method of operating an ultra-high frequency discharge device ofthe type in which transit time effects play a controlling part andhaving means for projecting a stream of charges along a prescribed path,a signal input electrode defining an input region, an intermediateelectrode, a signal output electrode defining an output region, saidelectrodes being disposed in succession along said path, and means forapplying bias potentials to said electrodes, which comprises adjustingthe steady potential of said input electrode and the density of saidstream current to values such that space charge in said input region issubstantially complete, adjusting the potential of said output electrodeto a value such that space charge in said output region isinsubstantial, imparting signal frequency variations to the current ofsaid stream to cause said charges to become grouped in the course oftheir passage to said output region, and adjusting the potential of saidintermediate electrode to a value such as to produce in the vicinity ofsaid intermediate electrode a space charge of a high value short ofcompleteness, whereby the density of said charge groups at said outputregion is enhanced.

5. In an ultra-high frequency discharge device of the type in whichtransit time effects play a controlling part, the combination whichcomprises means for projecting a stream of charges along a prescribedpath, signal input means defining .an input region disposed along saidpath for imparting signal frequency velocity variations to the chargesof said stream, independent signal input means disposed along said pathfor imparting signal frequency charge density variations to said stream,means for adjusting the potential of said first-named input means to avalue such that the charge transit angle across said input region has adesired value, means for adjusting the phase displacement between saidimparted velocity variations and said imparted density variations to adesired value such that the effects of both of said variations combinesubstantially in phase to produce dense groups of moving charges at acertain point along the path of said projected beam, and means disposedat said point for abstracting signal frequency energy from said groups.r

6. The method of operating an ultra-high frequency discharge devicehaving in the order named a cathode, a control electrode. a screenelectrode and an anode, an energy input circuit connected between saidcathode and said control electrode and an energy output circuitconnected between said screen electrode and said anode, and means forapplying bias potentials to said electrodes, which comprises adjustingthe bias potential applied to said control electrode to a value such asto produce a favorable transit angle across the space between saidcathode and said control electrode and substantially complete spacecharge in said space, adjusting the potential of said screen electrodeuntil the absolute value of the expression is greater than unity, wherew is the mean electron speed at the plane of the control electrode, onis the mean electron speed at the plane of the screen, 01 is the chargetransit angle from the cathode to the control electrode, 92 is thecharge transit angle from the control electrode to the screen, 6 is thebase of natural logarithms, and

i=V-1, and g is a factor which measures the amount of space charge inthe region between said control electrode and said screen electrode,defined by where T is the actual transit time across said region and Tois the transit time in the absence of space charge, both measured whenno alternating currents are flowing, and adjusting the potential of saidanode toa value such that the transit angle across the region betweensaid screen and said anode is a'small fraction of a signal frequencycycle and the space charge within said region has a low value.

'7. In an electron discharge apparatus which comprises, in the ordernamed, a source of an electron stream, a signal input electrode coupledto said stream for imparting velocity variations to the electrons of thestream, a drift space in which said electrons tend, by reason of saidimparted velocity variations, to become grouped, an output electrode forabstracting energy of said charge groups, and a collector of spentelectrons, a source of potential connected to said input electrode andarranged to maintain said input electrode at a substantial positivepotential with respect to said stream source, and a source of potentialconnected to said output electrode and arranged to maintain said outputelectrode at a small positive potential with respect to said streamsource and at a substantial negative potential with respect to saidinput electrode, said negative potential operating to progressivelyincrease the density of said stream as it traverses said drift space,whereby the grouping of said charges as they pass said output electrodeis accentuated.

LISS C. PETERSON.

REFERENCES crrm) The following references are of record in, the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,281,935 Hansen et a1. May 5,1942 2,379,819 Mason July 3, 1945

